During rainstorms, water that is not absorbed into the ground runs off into storm sewer systems for delivery into freshwater systems such as streams, rivers, lakes, and wetlands. While flowing across parking lots, landscaped areas, and other surfaces, the storm-water runoff picks up debris and pollutants and carries them into the storm sewer systems. Particularly large amounts of pollutants are picked up at shopping centers with large parking lots, oil-change and auto-repair shops, gas stations, and so forth. These pollutants include motor oil and other hydrocarbons, particulate matter such as sand and grit, and miscellaneous debris such as vegetative matter, paper, plastic, and foam cups. For example, about 200 pounds of miscellaneous debris and 500 pounds of sand and grit is commonly carried off by storm-water runoff from some one-acre parking lots in 90 days.
To maintain freshwater systems, most cities and counties have regulations requiring that some of the pollutants be removed from the storm-water runoff before entering their storm sewer systems. In order to meet these regulations, facilities typically install on-site pollution traps to filter the storm-water runoff. These pollution traps are sometimes referred to as “oil/grit separators.”
Most conventional pollution traps provide only “first flush” filtration during the typical local storm event, but permit bypassing the filtration stage for larger storms. In fact, many jurisdictions require bypassing, some even at typical storm water flows. Bypassing filtration is a problem because most pollutants are more easily picked up and transported by storm water during higher flow periods. Unfortunately, just when the traps are needed most, a lot of pollutants bypass them and are delivered into the storm sewer systems. And most pollution traps that do not provide for bypassing accommodate the larger flows because they are oversized, which adds significantly to the cost to build, install, and maintain them.
Another problem with many pollution traps is they simply filter the storm water at the natural flow rate of the storm water passing through it. The faster the storm water flows through the trap, the less particulate matter pollutants can settle in the trap. Some other traps detain the storm water for a brief time to allow some of the particulate matter to settle. But these traps only detain the water for a few minutes at most, and even a small water flow will cause the particles to be re-suspended in the water. Therefore, these pollution traps allow a lot of particulate matter pollutants to pass though them, even before bypass occurs.
In addition, the filtering systems of some pollution traps include screens for capturing miscellaneous debris. These screens are typically partially submerged in the water in the middle of the trap so that the debris is always floating in the water. Because the debris is always floating, it does not block the screen. The problem with this configuration is that vegetation, paper, and other absorbent miscellaneous debris tends to become waterlogged, rot, and deteriorate into smaller parts. These small parts then pass through the screen, are re-suspended in the water, and are carried out of the trap. Moreover, vegetative matter contains nitrogen and phosphorus and carries other pollutants such as fertilizer, pesticides, and oils. And paper products carry inks and other surface adherents. So now these additional pollutants also pass through the screen with the deteriorated debris and out of the trap.
Accordingly, it can be seen that a need remains for a pollution trap that stays on-line and filters all the storm-water runoff from a parcel of land, without bypassing filtration or overflowing during larger-than-typical storms. In addition, there is needed a pollution trap that better induces settling of particulate matter and reduces waterlogging of absorbent miscellaneous debris, to provide improved filtration of pollutants from the storm water. Furthermore, a need exists for such a pollution trap that is cost-efficient to build, install, and maintain. It is to the provision of a pollution trap meeting these and other needs that the present invention is primarily directed.